<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    *{
        margin: 0;
        padding: 0;

    }
    .box{
        width: 100%;
        height: calc(100vh - 80vh);
        background-color: aqua;
    }
    .box1{
        height: calc(100vh - 20vh);
        width: 100px;
        background-color: black;
    }
</style>
<body>
    <div class="box"></div>
    <div class="box1"></div>
</body>
</html>